To obtain an image with less noise upon picking up a still image in an image pickup apparatus such as a digital camera, it is effective to ensure a sufficient exposure time. However, if the exposure time is extended, there is a problem of blurring an image and making a picked-up image unclear due to a movement of a camera by a camera shake and due to a movement of an object.
An electronic blur correction method has been proposed as a method for dealing with such a blur. For example, JP9-261526A discloses a method for obtaining a good blur-free image by performing a synthesizing process after successively performing an image pickup operation in a short exposure time to cause a little blur a plurality of times and performing a position adjustment process so that motions between a plurality of obtained images are canceled.
If a synthesizing process is performed in a region where a position adjustment process failed, there is a problem of image blurring or double image. For this problem, a method in which a difference value between corresponding pixels is calculated before an addition process (averaging process) by a synthesizing process and a position adjustment process is judged to have failed and the synthesizing process is not performed when the difference value exceeds a threshold is proposed in JP2002-290817A.
Similarly, a method for suppressing the occurrence of a blur and a double image by adjusting a weight of a weighted averaging process in a synthesizing process based on a difference value between corresponding pixels is proposed in JP2008-99260A. By adjusting a synthesis weight by pixel in this way, it is possible to suppress the occurrence of a problem in a region where a position adjustment failed (pixel selection process).
However, an SN improvement effect by synthesis varies by pixel in these methods, with the result that an image with varying noise is generated (noise nonuniformity).
A method for dealing with this problem is proposed in JP2009-194700A. The method disclosed in JP2009-194700A is described with reference to FIG. 15.
An image is picked up by an image pickup device 801 through an optical system 800 constructed by a lens or the like and stored in a frame memory 802. By repeating the above image pickup process a specified number of times, a plurality of images are stored in the frame memory 802. A motion information acquiring unit 803 detects motion vectors between the plurality of images stored in the frame memory 802 and outputs the motion vectors as motion information to a synthesizing unit 804. The synthesizing unit 804 additively synthesizes the plurality of images stored in the frame memory 802 after adjusting the positions thereof using the motion information. In the additive synthesis, a process is performed to exclude pixels unsuitable for synthesis and outputs the process result as synthesis number information together with a synthesized image. A parameter control unit 805 outputs a noise reduction parameter at each pixel to a noise reducing unit 806, considering required noise reduction strength at each pixel from the synthesis number information. The noise reducing unit 806 performs a noise reduction process on each pixel with the noise reduction parameter designated by the parameter control unit 805 for the input synthesized image and outputs the resultant to an image processing unit 807. The image processing unit 807 generates an output image by performing necessary image processings such as a color processing and a gradation conversion processing. By changing the noise reduction strength by pixel according to a variation of a synthesis number in this way, it becomes possible to synthesize images while suppressing the occurrence of noise nonuniformity.